In order to register for a service delivered by a server, a user may be asked to pass a test in order to prevent robots from registering automatically. Such a test may be a “captcha” (Acronym of “Completely Automated Public Turing test to Tell Computers and Humans Apart”) test, which is an automated “Turing” test to distinguish computers from humans. More specifically, such a test may ask a human to decipher a short fragment of text whose image is distorted and contaminated with graphical noise.
Different types of “captcha” tests have been developed, for example using questions about images or hard-to-read text. However, hackers have developed tools to pass such tests automatically, such as by using text recognition tools, “speech to text” conversion tools, and image recognition algorithms.
Currently, there are three types of hacks that can be used to trick “captcha” tests:                solving by automatic robots,        solving by humans paid for that purpose,        solving by a human who is working unintentionally by means of a hacker site that secretly uses the user by redirecting to that person a “captcha” test for him or her to solve.        
In the last situation, the user solves a “captcha” test without knowing that he or she is providing an answer to a hacker site that may then use that answer to register with a service delivered by a server.
There is a need to counter this sort of attack performed by means of a hacker site, which is located between the terminal of a user and a server to which the terminal is connected, and which can intercept communications between the terminal and the server.